The present invention relates to an image forming apparatus such as a copying machine, printer, facsimile machine or the like of an electrophotographic type, more particularly to an apparatus wherein a charge member contacted to the image bearing member to electrically charge the image bearing member.
The image formation process in an electrophotographic apparatus includes a uniform charging step of electrically charging an electrophotographic photosensitive member (photosensitive drum) to a predetermined uniform potential, as is well known. With an example of charging means for this purpose, a charge member in the form of a roller (charging roller) is contacted to the surface of the photosensitive drum, and the charging roller is supplied with a charging bias (a voltage in the form of superimposing DC high voltage and sine wave AC high voltage) It is empirically known that discharge current is preferably not lower than a predetermined level in order to provide a stabilized charging.
When the output voltage (sine wave AC voltage (Vo)) as shown in FIG. 19, for example, is applied to the charging roller from the high voltage source, a current having the same phase as the AC voltage (Vo), that is, a resistance load current through a resistance load between the charging roller and the photosensitive drum, a current having the phase which is advanced by 90xc2x0 beyond the AC voltage (Vo), that is a capacity load current through a capacity load between the charging roller and the photosensitive drum, a pulse current flowing at the peak of the amplitude of the AC voltage (Vo), that is, a discharge current between the charging roller and the photosensitive drum. In total, the waveform of the outputing current is as indicated by Io. Designated by Im is a detected current waveform of the AC current attracted to the high voltage source from the charging roller.
FIG. 20 shows a relation between the amplitude of the AC voltage (the output voltage) and the outputing current (Io). When the amplitude of the AC voltage is gradually increased, the amplitude of the AC voltage and the outputing current are substantially proportional to each other as long as the voltage amplitude is lower than a predetermined level. As shown in FIG. 19, this is because a resistance load current (Izr) and a capacity load current (Izc) are proportional to the voltage amplitude, and discharge phenomenon does not occur because the voltage amplitude is small, which means that no discharge current (Is) flows. When the amplitude of the AC voltage (output voltage) is further increased, the discharge phenomenon occurs at the predetermined voltage amplitude (Vs), and the total outputing current (Io) does not satisfy the proportional relationship, and the discharge current (Is) alone increases.
Therefore, in the prior art, the peak value (Ip in FIG. 19) of the total outputing current is controlled at a predetermined level by a control system which will be described hereinafter, by which the discharge current (Is) is intended to be substantially constant.
FIG. 21 shows a charging bias control circuit for applying the charging bias voltage to the charging roller. As shown in this Figure, the charging roller 2 contacted to the photosensitive drum 1 is connected with a high voltage source 3 and a control device 4 for controlling the high voltage source 3. When the high voltage source 3 receives a clock pulse of a CPU5 of the control device 4, a transistor 8 switches through a pull-up resistor 6 and a base resistor 7 to produce a clock pulse having an amplitude corresponding to an output of an operational amplifier 11 connected with a pull-up resistor 9 through a diode 10.
When the amplitude of the clock pulse is large, the driving voltage amplitude of the sine wave inputted to the high voltage transformer 12 is also large, and as a result, the amplitude of the AC voltage outputted to the charging roller 2 is also large, the clock pulse is inputted to the filter circuit 32, which in turn produces a sine wave output having the central value of +12V. The output is inputted to a primary coil of the high voltage transformer 12 through a high voltage transformer drive, and a sine wave AC high voltage is produced at the secondary coil. One side of the secondary coil is connected with a DC high voltage generating circuit 46 through a resistor 45, and a charging bias voltage in the form of a superimposed DC high voltage and AC high voltage is supplied to the charging roller 2 through an output protection resistor 47.
The filter circuit 32 is constituted by fourth butterworth filter including resistors 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, capacitors 24, 25, 26, 27, 28, 29 and operational amplifiers 30, 31 and a primary high path filter. The high voltage transformer drive circuit 44 is constituted by resistors 33, 34, 35, 36, 37, 38, a capacitor 39, transistors 40, 41, 42 and a Zenorun-diode 43.
The current flowing into the high voltage source 3 from the charging roller 2 is detected by a high voltage capacitor 49 for separating the DC current of the peak current detection circuit 48 from the high voltage source 3 and a current monitoring resistor 50. More particularly, the peak voltage of the detected voltage is held by the diode 51 and the capacitor 52 so that peak current is detected.
The resistor 53 is a discharge resistor for the capacitor 52, and the diode 54 is for current discharge protection.
In order to control the current attracted from the charging roller 2 at a predetermined level, the output of the peak current detection circuit 48 is inputted to a xe2x80x9cxe2x88x92xe2x80x9d (negative) terminal or contact of the operational amplifier 11, and a reference voltage provided by the resisters 55 and 56 is inputted to a xe2x80x9c+xe2x80x9d (positive) terminal or contact, and the output terminal or contact of the operational amplifier 11 is connected to an emitter of the transistor 8 through the diode 10, so that amplitude of the clock pulse inputted to the circuit 32 is controlled.
In the above-described conventional example of the charging bias control, as shown in FIG. 22, a discharge start current I1 in an initial property e (initial stage of use) is not kept constant but reduces to a discharge start current I2 as shown in property f after use in the certain term, because of contamination of the charging roller 2 with toner or the like. The discharge current of the peak value Ip increases from Is0 to Is1.
Therefore, if the peak current is controlled to be constant, the discharge current g increases from Is0 to Is1 with the increase of the integrated number of output prints (number of the image formations, as shown In FIG. 23. With further increase of the number of output prints, it exceeds Is1.
On the other hand, as shown in FIG. 23, an amount of scrape of a photosensitive layer at the surface of the photosensitive drum 1 (deterioration of the photosensitive drum 1) increases proportionally to the discharge current, and as a result, the speed of the scrape acceleratedly increases. This has shortened the service life of the photosensitive drum 1.
Accordingly, it is a principal object of the present invention to provide an image forming apparatus in which deterioration of an image bearing member attributable to a discharge current is prevented while avoiding improper charging. According to an aspect of the present invention, there is provided an image forming apparatus comprising an image bearing member;
a charge member for electrically charging said image bearing member while contacting to said image bearing member; voltage applying means for applying an oscillating voltage including a component of AC voltage to said charge member; first detecting means for detecting an average of the AC current applied to said charge member from said voltage applying means; second detecting means for detection a value of the AC current corresponding to a peak of the AC voltage; and control means for effecting control such that when a detected current value of said detecting means is smaller than a first predetermined value, the detected current value of said first detecting means is at the first predetermined value, and when the detected current value of said first detecting means is larger than the first predetermined value, a current value of said second detecting means is at a second predetermined value.
According to another aspect of the present invention, there is provided an image forming apparatus comprising an image bearing member; a charge member for electrically charging said image bearing member while contacting to said image bearing member; voltage applying means for applying an oscillating voltage including a component of AC voltage to said charge member; detecting means for detecting an average of an AC current supplied to charge member from said voltage applying means in a voltage range wherein an absolute value of the AC voltage is not less than a predetermined value; and control means for effecting control such that average detected current value of said detecting means is at a predetermined value.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.